Add numeric type

[thekevinbrown]

We would like to use this library to generate PIN numbers, which users must type on a numeric keypad (a separate hardware device). As it stands, we would need to use:

cryptoRandomString({ length: 4, characters: '0123456789' });

everywhere we do this.

My concern about this approach is it's easy to accidentally delete one character in the character set, and we'd end up with something like:

cryptoRandomString({ length: 4, characters: '012345789' });

Which at a glance you wouldn't notice is not actually the full numeric set and exponentially decreases the security of the generated PINs. I'd prefer to do:

cryptoRandomString({ length: 4, type: 'numeric' });

As this seems more readable to me, as well as throwing an error at runtime if it's subtly broken instead of just silently becoming less secure. We also use Typescript, so we'd see the error in the type option immediately as well.

Edit: I also fixed a typo in a comment, forgot to mention that.

[sindresorhus]

Makes sense. Thanks for the PR. 👍